In coating apparatus of the curtain coating type, the moving support is coated by causing a free falling curtain of coating liquid to impinge onto the moving support to form a layer on said support. An apparatus is described and used in U.S. Pat. No. 3,508,947 wherein a multilayer composite of a plurality of distinct layers is formed on a slide hopper and caused to impinge onto an object or moving support to form a coated layer thereon. U.S. Pat. No. 3,508,947 particularly relates to the manufacture of multilayer photographic materials such as photographic film and paper.
In the coating of photographic products it is necessary to constrain the edges of the curtain to eliminate narrowing of the curtain and a reduction in coating width. It is desirable to have the edges of the curtain be internal to the edges of the film or paper base, henceforth this will be referred to as internal edging. Internal edging is preferable to the practice of maintaining a curtain wider than the base and coating over the edges of the base. However, the edge guides are solid surfaces which slow the coating liquids because of drag they produce. This reduction in velocity results in a significant penalty in the maximum coating speed attainable near the edge. The prior art teaches introducing a lubricating layer of water, or another low viscosity liquid, along the edge guide to reduce the drag and increase the velocity of the coating solutions in the curtain. This water layer or low viscosity liquid layer must, however, be removed in order to maintain acceptable coating latitude and quality and to avoid any penalty in speed for drying the edges. In the removal of the lubricating layer the velocity of the coating liquids must not be reduced in the vicinity of the edge if high speed coating is desired. The prior art teaches the use of a vertical slit connected to a vacuum source at the bottom of the edge guide as the means by which the lubricating water is removed. This is described in U.S. Pat. No. 4,830,887 which is incorporated by reference herein. This technique tends to slow down the coating liquids as the lubricating layer is being removed, hence reducing the maximum attainable coating speed at the edge. Also, some lubricating liquid may flow beyond the slit and not be captured.
Therefore, it is desirable to remove the lubricating liquid layer very abruptly giving the coating liquids near the edge guide very little opportunity to slow down. This maximizes the momentum of the coating liquids in the falling curtain and therefore, maximizes the attainable coating speed for the specific layer viscosities and flow rates being used. It is also desirable to ensure complete removal of the lubricating liquid. The present invention describes a method and apparatus in which the lubricating liquid layer is removed completely and very abruptly. This allows the coating speed of the curtain coating process to be maximized.